A new virus structure, that of bacteriophage T7, has been determined by image processing of electron micrographs. We analyzed T7 polycapsid tubes because these structures are more amenable to image processing. Optical diffraction revealed that the polycapsids were based on cylindrical foldings of a hexagonal lattice with a spacing of 12.6 nm, which is similar to the lattice constant for other complex icosahedral phage capsids defined to date. However, the details of details of the T7 capsomer differ from the other results.